Exhaust purification device of internal combustion engine

ABSTRACT

An NO x  absorbent (18) is disposed in an exhaust passage of an internal combustion engine and the exhaust gas is constantly made to circulate through the NO x  absorbent (18) during the operation of the engine. The NO x  absorbent (18) absorbs the NO x  when the air-fuel ratio of the exhaust gas flowing into the NO x  absorbent (18) is lean and releases the absorbed NO x  when the air-fuel ratio of the exhaust gas flowing into the NO x  absorbent (18) becomes the stoichiometric air-fuel ratio or rich. In the majority of the engine operation region, the lean air-fuel mixture is burned in the combustion chamber (3), and the NO x  generated at this time is absorbed into the NO x  absorbent (18). The air-fuel ratio of the exhaust gas flowing into the NO x  absorbent (18) is periodically made the stoichiometric air-fuel ratio or rich, and the NO x  absorbed in the NO x  absorbent (18) is released, and simultaneously reduced.

TECHNICAL FIELD

The present invention relates to a exhaust purification device of aninternal combustion engine.

BACKGROUND ART

A diesel engine in which an engine exhaust passage is branched to a pairof exhaust branch passages for purifying NO_(x), in which a switchingvalve is disposed at the branched portion of these exhaust branchpassages to alternately guide the exhaust gas to one of the exhaustbranch passages by a switching function of the switching valve, and inwhich a catalyst which can oxidize and absorb the NO_(x) is disposed ineach of the exhaust branch passages is well known (refer to JapaneseUnexamined Patent Publication No. 62-106826). In this diesel engine,NO_(x) in the exhaust gas introduced into one exhaust branch passage isoxidized and absorbed by the catalyst disposed in that exhaust branchpassage. During this time, the inflow of the exhaust gas to the otherexhaust branch passage is stopped and, at the same time, a gaseousreducing agent is fed into this exhaust branch passage. The NO_(x)accumulated in the catalyst disposed in this exhaust branch passage isreduced by this reducing agent. Subsequently, after a short time, theintroduction of the exhaust gas to the exhaust branch passage to whichthe exhaust gas had been introduced heretofore is stopped by theswitching function of the switching valve, and the introduction of theexhaust gas to the exhaust branch passage to which the introduction ofthe exhaust gas had been stopped heretofore is started again.

However, when the introduction of the exhaust gas to a pair of exhaustbranch passages is alternately stopped, the temperature of the catalystin the exhaust branch passage on the side where the introduction of theexhaust gas was stopped is gradually lowered in the period where theintroduction of the exhaust gas is stopped and is lowered to aconsiderably low temperature near the time when the introduction of theexhaust gas is started again. When the temperature of the catalystbecomes low in this way, there arises a problem in that the catalyticfunction of the catalyst is lowered, and therefore the oxidation andabsorption function of NO_(x) is not sufficiently carried out. In theperiod from when the introduction of the exhaust gas is started to whenthe catalyst temperature rises, the NO_(x) is not absorbed by thecatalyst and thus is discharged to the atmosphere.

Also, in this diesel engine, a pair of exhaust branch passages must beprovided, and a switching valve becomes necessary. Therefore, theconstruction becomes complex. Further, the switching valve is alwaysexposed to the high temperature exhaust gas, and therefore there arisesa problem of durability of the switching valve. Also, from the viewpointof the absorption of NO_(x), one catalyst is always idle, and thereforethere is another problem such that the entire catalyst which is providedis not effectively utilized for the absorption of NO_(x).

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purificationdevice which can efficiently absorb NO_(x) without a complexconstruction of the exhaust system and can release the absorbed NO_(x)according to need.

According to the present invention, there is provided an exhaustpurification device of an internal combustion engine wherein an NO_(x)absorbent which absorbs the NO_(x) when an air-fuel ratio of the exhaustgas flowing into the NO_(x) absorbent is lean, and which releases theabsorbed NO_(x) when the oxygen concentration in the exhaust gas flowinginto the NO_(x) absorbent is lowered is disposed in an engine exhaustpassage, the exhaust gas continuously flows into the NO_(x) absorbentduring the operation of the engine, and the NO_(x) absorbed in theNO_(x) absorbent when the exhaust gas flowing into the NO_(x) absorbentis lean is released from the NO_(x) absorbent when the oxygenconcentration in the exhaust gas flowing into the NO_(x) absorbent islowered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of an internal combustion engine;

FIG. 2 is a diagram showing a map of a basic fuel injection time;

FIG. 3 is a diagram showing a change of a correction coefficient K;

FIG. 4 is a graph schematically showing the concentration of unburnt HCand CO in the exhaust and oxygen discharged from the engine;

FIG. 5(A)-5(B) is a diagram for explaining an absorption and releasingoperation of the NO_(x) ;

FIG. 6 is a diagram showing an absorption rate of NO_(x) ;

FIG. 7(A)-7(B) is a diagram showing a control of the air-fuel ratio;

FIG. 8 is a flow chart showing an interruption routine;

FIG. 9 is a flow chart for calculating a fuel injection time TAU;

FIG. 10 is an overall view showing another embodiment of the internalcombustion engine;

FIG. 11 is a graph showing an output of the air-fuel ratio sensor;

FIG. 12 is a flow chart for calculating a feedback correctioncoefficient F;

FIG. 13 is a flow chart for calculating the fuel injection time TAU;

FIG. 14 is an overall view showing still another embodiment of theinternal combustion engine;

FIG. 15 is an overall view showing still another embodiment of theinternal combustion engine;

FIG. 16 is an overall view showing further still another embodiment ofthe internal combustion engine;

FIG. 17 is a flow chart showing an interruption routine;

FIG. 18 is a flow chart showing a main routine;

FIG. 19 is an overall view showing furthermore still another embodimentof the internal combustion engine; and

FIG. 20 is a flow chart for performing the NO_(x) releasing processing.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a case where the present invention is applied to a gasolineengine.

Referring to FIG. 1, 1 denotes an engine body; 2 a piston; 3 acombustion chamber; 4 a spark plug; 5 an intake valve; 6 an intake port;7 an exhaust valve; and 8 an exhaust port, respectively. The intake port6 is connected to a surge tank 10 via a corresponding branch pipe 9, anda fuel injector 11 injecting the fuel toward the interior of the intakeport 6 is attached to each branch pipe 9. The surge tank 10 is connectedto an air cleaner 14 via an intake duct 12 and an air flow meter 13, anda throttle valve 15 is disposed in the intake duct 12. On the otherhand, the exhaust port 8 is connected via an exhaust manifold 16 and anexhaust pipe 17 to a casing 19 including the NO_(x) absorbent 18therein.

An electronic control unit 30 comprises a digital computer and isprovided with a ROM (read only memory) 32, a RAM (random access memory)33, a CPU (microprocessor) 34, an input port 35, and an output port 36,which are interconnected by a bidirectional bus 31. The air flow meter13 generates an output voltage proportional to the amount of intake air,and this output voltage is input via an AD converter 37 to the inputport 35. A temperature sensor 20 generating an output voltageproportional to the exhaust temperature is attached in the exhaust pipe17 upstream of the casing 19, and the output voltage of this temperaturesensor 20 is input via the AD converter 38 to the input port 35. Also,an engine speed sensor 21 generating an output pulse expressing theengine speed is connected to the input port 35. On the other hand, theoutput port 36 is connected via the corresponding driving circuits 39and 40 to the spark plug 4 and fuel injector 11, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injectiontime TAU is calculated based on for example the following equation.

    TAU=TP·K

where, TP is a basic fuel injection time, and K is a correctioncoefficient. The basic fuel injection time TP shows the fuel injectiontime necessary for bringing the air-fuel ratio of an air-fuel mixturefed into the engine cylinder to the stoichiometric air-fuel ratio. Thisbasic fuel injection time TP is found in advance by experiments and isstored in advance in the ROM 32 in the form of a map as shown in FIG. 2as the function of an engine load Q/N (intake air amount Q/engine speedN) and the engine speed N. The correction coefficient K is a coefficientfor controlling the air-fuel ratio of the air-fuel mixture fed into theengine cylinder, and if K=1.0, the air-fuel ratio of the air-fuelmixture fed into the engine cylinder becomes the stoichiometric air-fuelratio. Contrary to this, when K becomes smaller than 1.0, the air-fuelratio of the air-fuel mixture fed into the engine cylinder becomeslarger than the stoichiometric air-fuel ratio, that is, becomes lean,and when K becomes larger than 1.0, the air-fuel ratio of the air-fuelmixture fed into the engine cylinder becomes smaller than thestoichiometric air-fuel ratio, that is, becomes rich.

This correction coefficient K is controlled in accordance with theoperating state of the engine. FIG. 3 shows one embodiment of thecontrol of this correction coefficient K. In the embodiment shown inFIG. 3, during a warm-up operation, the correction coefficient K isgradually lowered as the engine cooling water temperature becomeshigher. When the warm-up is completed, the correction coefficient K ismaintained at a constant value smaller than 1.0, that is, the air-fuelratio of the air-fuel mixture fed into the engine cylinder is maintainedas lean. Subsequently, when an acceleration operation is carried out,the correction coefficient K is brought to, for example, 1.0, that is,the air-fuel ratio of the air-fuel mixture fed into the engine cylinderis brought to the stoichiometric air-fuel ratio. When a full loadoperation is carried out, the correction coefficient K is made largerthan 1.0. Namely, the air-fuel ratio of the air-fuel mixture fed intothe engine cylinder is made rich. As seen from FIG. 3, in the embodimentshown in FIG. 3, except for the time of the warm-up operation, the timeof the acceleration operation, and the time of the full load operation,the air-fuel ratio of the air-fuel mixture fed into the engine cylinderis maintained at a constant lean air-fuel ratio, and accordingly thelean air-fuel mixture is burned in a majority of the engine operationregion.

FIG. 4 schematically shows the concentration of representativecomponents in the exhaust gas discharged from the combustion chamber 3.As seen from FIG. 4, the concentration of the unburnt HC and CO in theexhaust gas discharged from the combustion chamber 3 is increased as theair-fuel ratio of the air-fuel mixture fed into the combustion chamber 3becomes richer, and the concentration of the oxygen O₂ in the exhaustgas discharged from the combustion chamber 3 is increased as theair-fuel ratio of the air-fuel mixture fed into the combustion chamber 3becomes leaner.

The NO_(x) absorbent 18 contained in the casing 19 uses, for example,alumina as a carrier. On this carrier, at least one substance selectedfrom alkali metals, for example, potassium K, sodium Na, lithium Li, andcesium Cs; alkali earth metals, for example, barium Ba and calcium Ca;rare earth metals, for example, lanthanum La and yttrium Y; and preciousmetals such as platinum Pt, is carried. When referring to the ratiobetween the air and fuel (hydrocarbons) fed into the intake passage ofthe engine and the exhaust passage upstream of the NO_(x) absorbent 18as the air-fuel ratio of the exhaust gas flowing into the NO_(x)absorbent to the NO_(x) absorbent 18, this NO_(x) absorbent 18 performsthe absorption and releasing operation of NO_(x) by absorbing the NO_(x)when the air-fuel ratio of the exhaust gas flowing into the NO_(x)absorbent is lean, while releasing the absorbed NO_(x) when theconcentration of oxygen in the exhaust gas flowing into the NO_(x)absorbent falls. Note that, where the fuel (hydrocarbons) or air is notfed into the exhaust passage upstream of the NO_(x) absorbent 18, theair-fuel ratio of the exhaust gas flowing into the NO_(x) absorbentcoincides with the air-fuel ratio of the air-fuel mixture fed into thecombustion chamber 3, and accordingly in this case, the NO_(x) absorbent18 absorbs the NO_(x) when the air-fuel ratio of the air-fuel mixturefed into the combustion chamber 3 is lean and releases the absorbedNO_(x) when the concentration of oxygen in the air-fuel mixture fed intothe combustion chamber 3 is lowered.

When the above-mentioned NO_(x) absorbent 18 is disposed in the exhaustpassage of the engine, this NO_(x) absorbent 18 actually performs theabsorption and releasing operation of NO_(x), but there are areas of theexact mechanism of this absorption and releasing operation which are notclear. However, it can be considered that this absorption and releasingoperation is conducted by the mechanism as shown in FIG. 5. Thismechanism will be explained by using as an example a case where platinumPt and barium Ba are carried on the carrier, but a similar mechanism isobtained even if another precious metal, alkali metal, alkali earthmetal, or rare earth metal is used.

Namely, when the exhaust gas flowing into the NO_(x) absorbent becomesconsiderably lean, the concentration of oxygen in the exhaust gasflowing into the NO_(x) absorbent is greatly increased. As shown in FIG.5(A), the oxygen O₂ is deposited on the surface of the platinum Pt inthe form of O₂ ⁻. On the other hand, the NO in the exhaust gas flowinginto the NO_(x) absorbent reacts with the O₂ ⁻ on the surface of theplatinum Pt and becomes NO₂ (2NO+O₂ →2NO₂). Subsequently, a part of theproduced NO₂ is oxidized on the platinum Pt and absorbed into theabsorbent. While bonding with the barium oxide BaO, it is diffused inthe absorbent in the form of nitric acid ions NO₃ ⁻ as shown in FIG.5(A). In this way, NO_(x) is absorbed into the NO_(x) absorbent 18.

So long as the oxygen concentration in the exhaust gas flowing into theNO_(x) absorbent is high, the NO_(x) is produced on the surface of theplatinum Pt, and so long as the NO_(x) absorption ability of theabsorbent is not saturated, the NO_(x) is absorbed into the absorbentand nitric acid ions NO₃ ⁻ are produced. Contrary to this, when theoxygen concentration in the exhaust gas flowing into the NO_(x)absorbent is lowered and the production of NO₂ is lowered, the reactionproceeds in an inverse direction (NO₃ ⁻ →NO₂), and thus nitric acid ionsNO₃ ⁻ in the absorbent are released in the form of NO₂ from theabsorbent. Namely, when the oxygen concentration in the exhaust gasflowing into the NO_(x) absorbent is lowered, the NO_(x) is releasedfrom the NO_(x) absorbent 18. As shown in FIG. 4, when the degree ofleanness of the exhaust gas flowing into the NO_(x) absorbent becomeslow, the oxygen concentration in the exhaust gas flowing into the NO_(x)absorbent is lowered, and accordingly when the degree of leanness of theexhaust gas flowing into the NO_(x) absorbent is lowered, the NO_(x) isreleased from the NO_(x) absorbent 18 even if the air-fuel ratio of theexhaust gas flowing into the NO_(x) absorbent is lean.

On the other hand, at this time, when the air-fuel ratio of the air-fuelmixture fed into the combustion chamber 3 is made rich and the air-fuelratio of the exhaust gas flowing into the NO_(x) absorbent becomes rich,as shown in FIG. 4, a large amount of unburnt HC and CO is dischargedfrom the engine, and these unburnt HC and CO react with the oxygen O₂ ⁻on the platinum Pt and are oxidized. Also, when the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) absorbent becomes rich, theoxygen concentration in the exhaust gas flowing into the NO_(x)absorbent is extremely lowered, and therefore the NO₂ is released fromthe absorbent. This NO₂ reacts with the unburnt HC and CO as shown inFIG. 5(B) and is reduced. In this way, when the NO₂ no longer exists onthe surface of the platinum Pt, the NO₂ is successively released fromthe absorbent. Accordingly, when the air-fuel ratio of the exhaust gasflowing into the NO_(x) absorbent is made rich, the NO_(x) is releasedfrom the NO_(x) absorbent 18 in a short time.

Namely, when the air-fuel ratio of the exhaust gas flowing into theNO_(x) absorbent is made rich, first of all, the unburnt HC and COimmediately react with the O₂ ⁻ on the platinum Pt and are oxidized, andsubsequently if the unburnt HC and CO still remain even though the O₂ ⁻on the platinum Pt is consumed, the NO_(x) released from the absorbentand the NO_(x) discharged from the engine are reduced by these unburntHC and CO. Accordingly, when the air-fuel ratio of the exhaust gasflowing into the NO_(x) absorbent is made rich, the NO_(x) absorbed inthe NO_(x) absorbent 18 is released in a short time and in addition thisreleased NO_(x) is reduced, and therefore the discharge of NO_(x) intothe atmosphere can be blocked. Also, since the NO_(x) absorbent 18 hasthe function of a reduction catalyst, even if the air-fuel ratio of theexhaust gas flowing into the NO_(x) absorbent is made the stoichiometricair-fuel ratio, the NO_(x) released from the NO_(x) absorbent 18 can bereduced. However, where the air-fuel ratio of the exhaust gas flowinginto the NO_(x) absorbent is made the stoichiometric air-fuel ratio, theNO_(x) is released merely gradually from the NO_(x) absorbent 18, andtherefore a slightly long time is required for releasing all NO_(x)absorbed in the NO_(x) absorbent 18.

When the degree of leanness of the exhaust gas flowing into the NO_(x)absorbent is lowered as mentioned before, even if the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) absorbent is lean, the NO_(x) isreleased from the NO_(x) absorbent 18. Accordingly, to release theNO_(x) from the NO_(x) absorbent 18, it is satisfactory if theconcentration of oxygen in the exhaust gas flowing into the NO_(x)absorbent is lowered. Note, even if the NO_(x) is released from theNO_(x) absorbent 18, when the air-fuel ratio of the exhaust gas flowinginto the NO_(x) absorbent is lean, the NO_(x) is not reduced in theNO_(x) absorbent 18, and accordingly, in this case, it is necessary toprovide a catalyst which can reduce the NO_(x) downstream of the NO_(x)absorbent 18 or supply a reducing agent downstream of the NO_(x)absorbent 18. Of course, it is also possible to reduce the NO_(x)downstream of the NO_(x) absorbent 18 in this way, but it is ratherpreferable that the NO_(x) be reduced in the NO_(x) absorbent 18.Accordingly, in the embodiment according to the present invention, whenthe NO_(x) should be released from the NO_(x) absorbent 18, the air-fuelratio of the exhaust gas flowing into the NO_(x) absorbent is made thestoichiometric air-fuel ratio or rich, whereby the NO_(x) released fromthe NO_(x) absorbent 18 is reduced in the NO_(x) absorbent 18.

FIG. 6 shows the absorption rate R of the NO_(x) absorbed into theNO_(x) absorbent 18 when the air-fuel ratio of the exhaust gas flowinginto the NO_(x) absorbent is lean. Note that, the abscissa T shows thetemperature of the NO_(x) absorbent 18. In actuality, the temperature Tof the NO_(x) absorbent 18 becomes almost equal to the temperature ofthe exhaust gas flowing into the NO_(x) absorbent 18. As seen from FIG.6, when the temperature of the NO_(x) absorbent 18 becomes lower thanabout 200° C. indicated by T₁, the oxidation function of NO_(x) (2NO+O₂→2NO₂) is weakened, and therefore the NO_(x) absorption rate R islowered. Moreover, at this time, also the releasing operation of NO_(x)(NO₃ ⁻ →NO₂) is weakened, and therefore even if the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) absorbent is made thestoichiometric air-fuel ratio or rich, it becomes impossible to releasethe NO_(x) from the NO_(x) absorbent 18 well. On the other hand, whenthe temperature T of the NO_(x) absorbent 18 becomes higher than about500° C. indicated by T₂, the NO_(x) absorbed in the NO_(x) absorbent 18is decomposed and naturally released from the NO_(x) absorbent 18, andtherefore the NO_(x) absorption rate is lowered. Accordingly, the NO_(x)is absorbed well into the NO_(x) absorbent 18 when the temperature T ofthe NO_(x) absorbent 18 is within the predetermined temperature range(T₁ <T<T₂).

As shown in FIG. 3, in the embodiment according to the presentinvention, the air-fuel ratio of the air-fuel mixture fed into thecombustion chamber 3 is made rich at the time of the warm-up operationand at the time of the full load operation, and the air-fuel ratio ismade the stoichiometric air-fuel ratio at the time of the accelerationoperation, but the lean air-fuel mixture is burned in the combustionchamber 3 in the majority of the operation region other than these. Inthis case, the air-fuel ratio of the air-fuel mixture burned in thecombustion chamber 3 is more than 18.0. In the embodiment shown in FIG.1, a lean air-fuel mixture having an air-fuel ratio of from about 20 to24 is burned. When the air-fuel ratio becomes more than 18.0, even ifthe three-way catalyst has a reduction property under a lean air-fuelratio, it cannot sufficiently reduce the NO_(x), and accordingly thethree-way catalyst cannot be used so as to reduce the NO_(x) under sucha lean air-fuel ratio. Also, as a catalyst which can reduce the NO_(x)even if the air-fuel ratio is more than 18.0, there is a Cu-zeolitecatalyst, but this Cu-zeolite catalyst lacks heat resistance, andtherefore the use of this Cu-zeolite catalyst is not preferable inpractice. Accordingly, in the end, there is no method of purifying theNO_(x) when the air-fuel ratio is more than 18.0 other than the methodof using the NO_(x) absorbent 18 which is used in the present invention.

In the embodiment according to the present invention, as mentionedabove, the air-fuel ratio of the air-fuel mixture fed into thecombustion chamber 3 is made rich at the time of the full loadoperation, and that of the air-fuel mixture is made the stoichiometricair-fuel ratio at the time of the acceleration operation, and thereforeNO_(x) is released from the NO_(x) absorbent 18 at the time of the fullload operation and at the time of the acceleration operation. However,when the frequency of such a full load operation or accelerationoperation is low, even if the NO_(x) is released from the NO_(x)absorbent 18 only at the time of the full load operation andacceleration operation, the absorption ability of the NO_(x) by theNO_(x) absorbent 18 is saturated during the period where the leanair-fuel mixture is burned, and thus the NO_(x) is no longer absorbed bythe NO_(x) absorbent 18. Accordingly, in the embodiment according to thepresent invention, when the lean air-fuel mixture is continuouslyburned, as shown in FIG. 7(A), the air-fuel ratio of the exhaust gasflowing into the NO_(x) absorbent is periodically made rich, or theair-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent isperiodically made the stoichiometric air-fuel ratio as shown in FIG.7(B). Note that, in this case, as shown in FIG. 7(C), it is alsopossible to periodically lower the degree of leanness, but in this case,the NO_(x) is not reduced in the NO_(x) absorbent 18, and therefore, asmentioned before, the NO_(x) must be reduced downstream of the NO_(x)absorbent 18.

As shown in FIG. 7(A), looking at the case where the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) absorbent is periodically maderich, a time t₂ over which the air-fuel ratio of the exhaust gas flowinginto the NO_(x) absorbent is made rich is much shorter than the time t₁over which the combustion of the lean air-fuel mixture is carried out.Concretely speaking, while the time t₂ over which the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) absorbent is made rich is lessthan about 10 seconds, the time t₁ over which the combustion of the leanair-fuel mixture is carried out becomes a time of from 10 odd minutes toone hour or more. Namely, in other words, t₂ becomes 50 times or morelonger than t₁. This is true also in the cases shown in FIGS. 7(B) and7(C).

The releasing operation of the NO_(x) from the NO_(x) absorbent 18 iscarried out when a constant amount of NO_(x) is absorbed into the NO_(x)absorbent 18, for example when NO_(x) of 50% of the absorption abilityof the NO_(x) absorbent 18 is absorbed. The amount of NO_(x) absorbedinto the NO_(x) absorbent 18 is proportional to the amount of theexhaust gas discharged from the engine and the NO_(x) concentration inthe exhaust gas. In this case, the amount of the exhaust gas isproportional to the intake air amount, and the NO_(x) concentration inthe exhaust gas is proportional to the engine load, and therefore theamount of NO_(x) absorbed into the NO_(x) absorbent 18 is proportionalto the amount of intake air and the engine load. Accordingly, the amountof the NO_(x) absorbed in the NO_(x) absorbent 18 can be estimated fromthe cumulative value of the product of the amount of the intake air withthe engine load, but in the embodiment according to the presentinvention, it is simplified and the amount of NO_(x) absorbed in theNO_(x) absorbent 18 is estimated from the cumulative value of the enginespeed.

An explanation will be made next of one embodiment of absorption andreleasing control of the NO_(x) absorbent 18 according to the presentinvention with reference to FIG. 8 and FIG. 9.

FIG. 8 shows an interruption routine executed at predetermined timeintervals.

Referring to FIG. 8, first, it is judged at step 100 whether or not thecorrection coefficient K with respect to the basic fuel injection timeTP is smaller than 1.0, that is, whether or not the lean air-fuelmixture has been burned. When K<1.0, that is, when the lean air-fuelmixture has been burned, the processing routine goes to step 101, atwhich the result of addition of ΣNE to the current engine speed NE isdefined as ΣNE. Accordingly, this ΣNE indicates the cumulative value ofthe engine speed NE. Subsequently, at step 102, it is judged whether ornot the cumulative engine speed ΣNE is larger than the constant valueSNE. This constant value SNE shows a cumulative engine speed from whichit is estimated that NO_(x) in an amount of for example 50% of theabsorption ability of NO_(x) is absorbed by the NO_(x) absorbent 18.When ΣNE≦SNE, the processing cycle is completed, and when ΣNE>SNE, thatis, when it is estimated that NO_(x) in an amount of 50% of the NO_(x)absorption ability of the NO_(x) absorbent 18 is absorbed therein, theprocessing routine goes to step 103. At step 103, it is judged whetheror not the exhaust temperature T is lower than a constant value T₁, forexample, 200° C. When T<T₁, the processing cycle is completed, and whenT≧T₁, the processing routine goes to step 104, at which the NO_(x)releasing flag is set. When the NO_(x) releasing flag is set, as will bementioned later, the air-fuel ratio of the air-fuel mixture fed into theengine cylinder is made rich.

Subsequently, at step 105, the count value C is incremented exactly by"1". Subsequently, at step 106, it is judged whether or not the countvalue C becomes larger than a constant value C₀, that is, whether or notfor example five seconds have elapsed. When C≦C₀, the processing routineis completed, and when C becomes larger than C₀, the processing routinegoes to step 107, at which the NO_(x) releasing flag is reset. When theNO_(x) releasing flag is reset, as will be mentioned later, the air-fuelratio of the air-fuel mixture fed into the engine cylinder is switchedfrom rich to lean, and thus the air-fuel ratio of the air-fuel mixturefed into the engine cylinder is made rich for 5 seconds. Subsequently,at step 108, the cumulative engine speed ΣNE and the count value C arebrought to zero.

On the other hand, at step 100, when it is decided that K≧1.0, that is,when the air-fuel ratio of the air-fuel mixture fed into the enginecylinder is the stoichiometric air-fuel ratio or rich, the processingroutine goes to step 109, at which it is judged whether or not the stateof K≧1.0 is continued for a constant time, for example, 10 seconds. Whenthe state of K≧1.0 is not continued for the predetermined time, theprocessing cycle is completed, and when the state of K≧1.0 is continuedfor the predetermined time, the processing routine goes to step 110, atwhich the cumulative engine speed ΣNE is brought to zero.

Namely, when the time over which the air-fuel ratio of the air-fuelmixture fed into the engine cylinder is made the stoichiometric air-fuelratio or rich is continued for about 10 seconds, it can be consideredthat most of the NO_(x) absorbed in the NO_(x) absorbent 18 wasreleased, and accordingly in this case, the cumulative engine speed ΣNEis brought to zero at step 110. Also, at step 103, when T<T₁, even ifthe air-fuel ratio of the air-fuel mixture fed into the engine cylinderis made rich, the temperature of the NO_(x) absorbent 18 is low, andtherefore the NO_(x) is not released from the NO_(x) absorbent 18.Accordingly, when T<T₁, the processing is delayed until T becomes equalto or larger than T₁, and when T becomes equal to or larger than T₁, theair-fuel ratio of the air-fuel mixture fed into the engine cylinder ismade rich.

FIG. 9 shows a calculation routine of the fuel injection time TAU. Thisroutine is repeatedly executed.

Referring to FIG. 9, first, at step 200, a basic fuel injection time TPis calculated from a map indicated in FIG. 2. Subsequently, at step 201,it is judged whether or not the operation state is a state wherecombustion of the lean air-fuel mixture should be carried out. When itis not an operation state where combustion of the lean air-fuel mixtureshould be carried out, that is, at the time of the warm-up operation,acceleration operation, or full load operation, the processing routinegoes to step 202, at which the correction coefficient K is calculated.At the time of an engine warm-up operation, this correction coefficientK is a function of the engine cooling water temperature and becomessmaller as the engine cooling water temperature becomes higher within arange indicated by K≧1.0. Also, at the time of the accelerationoperation, the correction coefficient K is brought to 1.0, and at thetime of the full load operation, the correction coefficient K is made avalue larger than 1.0. Subsequently, at step 203, the correctioncoefficient K is made Kt, and subsequently, at step 204, the fuelinjection time TAU (=TP·Kt) is calculated. At this time, the air-fuelratio of the air-fuel mixture fed into the engine cylinder is made thestoichiometric air-fuel ratio or rich.

On the other hand, at step 201, when it is judged that the operationstate is a state where combustion of the lean air-fuel mixture should becarried out, the processing routine goes to step 205, at which it isjudged whether or not the NO_(x) releasing flag has been set. When theNO_(x) releasing flag has not been set, the processing routine goes tostep 206, at which the correction coefficient K is made for example 0.6,and subsequently, at step 207, the correction coefficient K is changedto Kt, and then the processing routine goes to step 204. Accordingly, atthis time, a lean air-fuel mixture is fed into the engine cylinder. Onthe other hand, when it is decided at step 205 that the NO_(x) releasingflag was set, the processing routine goes to step 208, at which thepreliminarily determined value KK is changed to Kt, and subsequently theprocessing routine goes to step 204. This value KK is a value of fromabout 1.1 to 1.2 with which the air-fuel ratio of the air-fuel mixturefed into the engine cylinder becomes about 12.0 to 13.5. Accordingly, atthis time, the rich air-fuel mixture is fed into the engine cylinder,whereby the NO_(x) absorbed in the NO_(x) absorbent 18 is released. Notethat, at the releasing of NO_(x), where the air-fuel mixture is to bemade the stoichiometric air-fuel ratio, the value of KK is brought to1.0.

FIG. 10 indicates another embodiment. In this embodiment, the sameconstituent elements as those shown in FIG. 1 are indicated by the samereference numerals.

As shown in FIG. 10, in this embodiment, an air-fuel ratio sensor 22which can detect the air-fuel ratio over a wide range is disposed in theexhaust manifold 16. This air-fuel ratio sensor 22 generates an outputvoltage V in accordance with the air-fuel ratio (A/F) as shown in FIG.11. Accordingly, the air-fuel ratio can be learned from this outputvoltage V. The output voltage V is input via the AD converter 41 to theinput port 35 as shown in FIG. 10.

In the embodiment indicated in FIG. 1, the value of the correctioncoefficient K is open loop controlled, and accordingly there is a riskthat the lean air-fuel ratio at the combustion of the lean air-fuelmixture and the rich air-fuel ratio at the releasing of NO_(x) willdeviate from the regular air-fuel ratios due to aging. In the embodimentshown in FIG. 10, the air-fuel ratio is subjected to feedback controlusing the air-fuel ratio sensor 22, whereby these lean air-fuel ratioand rich air-fuel ratio are always brought into coincidence with theregular air-fuel ratios.

Namely, as shown in FIG. 10, where the air-fuel ratio sensor 22 is used,the fuel injection time TAU is calculated based on the followingequation:

    TAU=TP·K·F·G

Here, the basic fuel injection time TP and the correction coefficient Kare the same as those used in the embodiment shown in FIG. 1 to FIG. 9,and a feedback correction coefficient F and a learning coefficient G arenewly added to this. This feedback correction coefficient F fluctuatesso that the air-fuel ratio coincides with the target air-fuel ratiobased on the output voltage V of the air-fuel ratio sensor 22, and thelearning coefficient G is changed so that a fluctuation around 1.0occurs. Note that, also in this embodiment, the routine shown in FIG. 8is used for controlling the NO_(x) releasing flag.

FIG. 12 shows a routine for calculating the feedback correctioncoefficient F, which routine is executed by interruption atpredetermined time intervals.

Referring to FIG. 12, first of all, at step 300, it is judged whether ornot the NO_(x) releasing flag is set. When the NO_(x) releasing flag isnot set, the processing routine goes to step 301, at which a targetair-fuel ratio (A/F)₀ corresponding to the correction coefficient K iscalculated. Subsequently, at step 302, the current air-fuel ratio (A/F)is calculated from the output voltage V of the air-fuel ratio sensor 22.Subsequently, at step 303, the target air-fuel ratio (A/F)₀ is comparedwith the present air-fuel ratio (A/F). When (A/F)₀ >(A/F), theprocessing routine goes to step 304, at which the constant value α issubtracted from the feedback correction coefficient F. As a result, thefuel injection time TAU is decreased, and therefore the air-fuel ratiobecomes larger. Contrary to this, when (A/F)₀ ≦(A/F), the processingroutine goes to step 305, at which the constant value α is added to thefeedback correction coefficient F. As a result, the fuel injection timeTAU is prolonged, and therefore the air-fuel ratio becomes smaller. Inthis way, the air-fuel ratio (A/F) is maintained at the target air-fuelratio (A/F)₀.

Subsequently, at step 306, the average value in the predetermined periodof the feedback correction coefficient F is defined as the learningcoefficient G. On the other hand, at step 300, when it is decided thatthe NO_(x) releasing flag is set, the processing routine goes to step307, at which the feedback correction coefficient F is fixed to 1.0.

FIG. 13 indicates a calculation routine of the fuel injection time TAU,which routine is repeatedly executed. This routine is the same as theroutine shown in FIG. 9 except for step 404.

Namely, referring to FIG. 13, first of all, at step 400, the basic fuelinjection time TP is calculated from the map shown in FIG. 2.Subsequently, at step 401, it is judged whether or not the operationstate is a state where combustion of the lean air-fuel mixture should becarried out. When the operation state is not a state where combustion ofthe lean air-fuel mixture should be carried out, that is, at the time ofthe warm-up operation, acceleration operation, or full load operation,the processing routine goes to step 402, at which the correctioncoefficient K is calculated. Subsequently, at step 403, the correctioncoefficient K is brought to Kt, and subsequently, at step 404, the fuelinjection time TAU (=TP·Kt·F·G) is calculated. At this time, theair-fuel ratio of the air-fuel mixture fed into the engine cylinder ismade to have the stoichiometric air-fuel ratio or rich air-fuel ratio.

On the other hand, when it is judged at step 401 that the operationstate is a state where combustion of the lean air-fuel mixture should becarried out, the processing routine goes to step 405, at which it isjudged whether or not the NO_(x) releasing flag is set. When the NO_(x)releasing flag is not set, the processing routine goes to step 406, atwhich the correction coefficient K is changed to, for example, 0.6, andsubsequently, after the correction coefficient K is brought to Kt atstep 407, the processing routine goes to step 404. Accordingly, at thistime, the lean air-fuel mixture is fed into the engine cylinder. On theother hand, when it is decided at step 405 that the NO_(x) releasingflag was set, the processing routine goes to step 408, at which thepreliminarily determined value KK is set to Kt, and subsequently, theprocessing routine goes to step 404. This value KK is a value of fromabout 1.1 to 1.2. Accordingly, at this time, a rich air-fuel mixture isfed into the engine cylinder, whereby the NO_(x) absorbed in the NO_(x)absorbent 18 is released.

As mentioned before, the learning coefficient G expresses an averagevalue of the feedback correction coefficient F in the predeterminedperiod. This feedback correction coefficient F originally fluctuatesaround 1.0. For example, when assuming that a deposit builds up in thenozzle port of the fuel injector 11, the feedback correction coefficientF becomes larger than 1.0 so as to maintain the air-fuel ratio (A/F) atthe target air-fuel ratio (A/F)₀. In this way, when the feedbackcorrection coefficient F becomes larger than 1.0, the learningcoefficient G becomes larger along with this, and thus the feedbackcorrection coefficient F always fluctuates around 1.0. Accordingly, inthis case, when the feedback correction coefficient F is fixed to 1.0,the air-fuel ratio (A/F) coincides with the target air-fuel ratio (A/F)₀corresponding to the correction coefficient K. In the embodiment shownin FIG. 10, as shown in FIG. 12, when the NO_(x) releasing flag is set,the feedback correction coefficient F is fixed to 1.0. Accordingly, atthis time, the air-fuel ratio of the air-fuel mixture fed into theengine cylinder is brought into a correct coincidence with the air-fuelratio corresponding to KK.

FIG. 14 shows still another embodiment. In this embodiment, an outputside of the casing 19 is connected via the exhaust pipe 23 with acatalytic converter 25 including a three-way catalyst 24 therein. Thisthree-way catalyst 24 exhibits a high purification efficiency withrespect to the CO, HC, and NO_(x) when the air-fuel ratio is maintainedat approximately the stoichiometric air-fuel ratio as is well known, butthis three-way catalyst 24 has a high purification efficiency withrespect to the NO_(x) even when the air-fuel ratio has become rich to acertain extent. In the embodiment shown in FIG. 14, a three-way catalyst24 is provided downstream of the NO_(x) absorbent 18 so as to purify theNO_(x) by utilizing this characteristic.

Namely, as mentioned before, when the air-fuel ratio of the air-fuelmixture fed into the engine cylinder is made rich so as to release theNO_(x) from the NO_(x) absorbent 18, the NO_(x) absorbed in the NO_(x)absorbent 18 is abruptly released from the NO_(x) absorbent 18. At thistime, although the NO_(x) is reduced at the releasing, there is apossibility that all the NO_(x) is not reduced. However, when thethree-way catalyst 24 is disposed downstream of the NO_(x) absorbent 18,the NO_(x) which was not reduced at the releasing is reduced by thethree-way catalyst 24. Accordingly, by disposing the three-way catalyst24 downstream of the NO_(x) absorbent 18, the NO_(x) purificationperformance can be further improved.

FIG. 15 shows more still another embodiment. In this embodiment, stillanother catalystic converter 27 including a three-way catalyst 26 isdisposed between the exhaust manifold 16 and the exhaust pipe 17. Inthis way, when the three-way catalyst 26 is disposed near the exhaustport 8, the three-way catalyst 26 is in contact with exhaust gas havinga higher temperature in comparison with the NO_(x) absorbent 18 and thethree-way catalyst 24, and therefore the three-way catalyst 26 abruptlyrises in its temperature after a start of the engine in comparison withthe NO_(x) absorbent 18 and the three-way catalyst 24. Accordingly, whenproviding such a three-way catalyst 26, it becomes possible to purifythe unburnt HC and CO generated in a large amount during the enginewarm-up operation by the three-way catalyst 26 from an early time afterthe start of the engine.

In the embodiments mentioned heretofore, as the NO_(x) absorbent, use ismade of an NO_(x) absorbent 18 in which at least one substance selectedfrom alkali metals, alkali earth metals, rare earth metals, and preciousmetals is carried on the alumina. However, it is possible to use acomposite oxide of an alkali earth metal with copper, that, is aBa--Cu--O system NO_(x) absorbent, instead of the use of such an NO_(x)absorbent 18. As such a composite oxide of the alkali earth metal withcopper, use can be made of, for example, MnO₂.BaCuO₂. In this case,platinum Pt or cerium Ce can be added.

In this MnO₂.BaCuO₂ system NO_(x) absorbent, the copper Cu performs thesame catalytic function as that of the platinum Pt of the NO_(x)absorbent 18 mentioned heretofore. When the air-fuel ratio is lean, theNO_(x) is oxidized by the copper Cu (2NO+O₂ →2NO₂) and diffused in theabsorbent in the form of the nitric acid ions NO₃ ⁻.

On the other hand, when the air-fuel ratio is made rich, similarly theNO_(x) is released from the absorbent, and this NO_(x) is reduced by thecatalytic function of the copper Cu. However, the NO_(x) reduction forceof the copper Cu is weaker in comparison with the NO_(x) reduction forceof the platinum Pt, and accordingly where the Ba--Cu--O system absorbentis used, an amount of NO_(x) which is not reduced at the releasing ofNO_(x) is slightly increased in comparison with the NO_(x) absorbent 18mentioned heretofore. Accordingly, where the Ba--Cu--O system absorbentis used, as shown in FIG. 14 and FIG. 15, preferably the three-waycatalyst 24 is disposed downstream of the absorbent.

FIG. 16 and FIG. 19 show a case where the present invention is appliedto a diesel engine. Note that, in FIG. 16 and FIG. 19, the sameconstituent elements as those in FIG. 1 are shown by the same referencenumerals.

In the diesel engine, usually, in all operation states, combustion iscarried out in a state where the excessive air ratio is more than 1.0,that is, the average air-fuel ratio of the air-fuel mixture in thecombustion chamber 3 is lean. Accordingly, the NO_(x) discharged at thistime is absorbed into the NO_(x) absorbent 18. On the other hand, whenthe NO_(x) should be released from the NO_(x) absorbent 18, the air-fuelratio of the exhaust gas flowing into the NO_(x) absorbent to the NO_(x)absorbent 18 is made rich. In this case, in the embodiment shown in FIG.16, the average air-fuel ratio of the air-fuel mixture in the combustionchamber 3 is made rich, whereby the air-fuel ratio of the exhaust gasflowing into the NO_(x) absorbent to the NO_(x) absorbent 18 is maderich. In the embodiment shown in FIG. 19, the average air-fuel ratio ofthe air-fuel mixture in the combustion chamber 3 is made lean, and thehydrocarbon is fed into the exhaust passage of engine upstream of theNO_(x) absorbent 18, whereby the air-fuel ratio of the exhaust gasflowing into the NO_(x) absorbent to the NO_(x) absorbent 18 is maderich.

Referring to FIG. 16, in this embodiment, a load sensor 51 generating anoutput voltage proportional to the amount of depression of theaccelerator pedal 51 is provided, and the output voltage of this loadsensor 51 is input via the AD converter 52 to the input port 35. Also,in this embodiment, a throttle valve 53 is disposed in the intake duct12, which throttle valve 53 is connected to a diaphragm 55 of a vacuumdiaphragm device 54. A diaphragm vacuum chamber 56 of the vacuumdiaphragm device 54 is selectively connected with the atmosphere or avacuum tank 58 via an electromagnetic switching valve 57, while theoutput port 36 of the electronic control unit 30 is connected to theelectromagnetic switching valve 57 via a driving circuit 59. For theelectromagnetic switching valve 57, a ratio between a time for which thediaphragm vacuum chamber 56 is communicated with the atmosphere and atime for which it is communicated with the vacuum tank 58, that is, theduty ratio DUTY, is controlled. As this duty ratio DUTY becomes larger,the opening degree of the throttle valve 53 becomes smaller.

In this embodiment, when the NO_(x) should be released from the NO_(x)absorbent 18, the amount of injection from the fuel injector 11 isincreased only by a constant amount ΔQ with respect to the requestedinjection amount with which the best combustion is obtained, andsimultaneously the throttle valve 53 is opened to the predeterminedopening degree so that the average air-fuel ratio of the air-fuelmixture in the combustion chamber 3 becomes rich. Namely, when theamount of injection from the fuel injector 11 is increased by only theconstant amount ΔQ with respect to the requested injection amount withwhich the best combustion is obtained, this increased amount worth ΔQ isnot burned well and is discharged to the interior of the exhaust port 8in the form of unburnt HC and CO. Also, at this time, the amount of airfed into the combustion chamber 3 is decreased by the opening operationof the throttle valve 53, and therefore the air-fuel ratio of theexhaust gas discharged to the interior of the exhaust port 8 becomerich. Accordingly, the air-fuel ratio of the exhaust gas flowing intothe NO_(x) absorbent flowing into the NO_(x) absorbent 18 becomes rich,and thus the NO_(x) is released from the NO_(x) absorbent 18. The amountΔQ of increase of fuel and amount of opening of the throttle valve 53when the NO_(x) should be released from the NO_(x) absorbent 18 arepreliminarily found by experiment.

FIG. 17 shows an interruption routine executed at predetermined timeintervals for executing the above-mentioned control.

Referring to FIG. 17, first of all, at step 500, a result obtained byadding ΣNE to the present engine speed NE is defined as ΣNE.Accordingly, this ΣNE indicates the cumulative value of the engine speedNE. Subsequently, at step 501, it is judged whether or not thecumulative engine speed ΣNE is larger than the predetermined value SNE.This predetermined value SNE indicates the cumulative engine speed fromwhich it is estimated that the NO_(x) in an amount of for example 50% ofthe NO_(x) absorption ability of the NO_(x) absorbent 18 is absorbedtherein. When ΣNE≦SNE, the processing cycle is completed, and whenΣNE>SNE, that is, when it is estimated that the NO_(x) in an amount of50% of the NO_(x) absorption ability of the NO_(x) absorbent 18 isabsorbed therein, the processing routine goes to step 502. At step 502,it is judged whether or not the exhaust temperature T is lower than thepredetermined value T₁, for example, 200° C. When T<T₁, the processingcycle is completed, and when T≧T₁, the processing routine goes to step503, at which the NO_(x) releasing flag is set. When the NO_(x)releasing flag is set, as will be mentioned later, the fuel injectionamount is increased, and the throttle valve 53 is opened to the constantopening degree.

Subsequently, at step 504, the count value C is incremented exactly by"1". Subsequently, at step 505, it is judged whether or not the countvalue C becomes larger than the predetermined value C₀, that is, whetheror not for example 5 seconds elapsed. When C≦C₀, the processing routineis completed, and when C becomes larger than C₀, the processing routinegoes to step 506, at which the NO_(x) releasing flag is reset. When theNO_(x) releasing flag is reset, as will be mentioned later, theincreasing operation of the fuel injection amount is stopped, and thethrottle valve 53 is fully opened. Accordingly, the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) absorbent 18 is made rich for 5seconds. Subsequently, at step 507, the cumulative engine speed ΣNE andthe count value C are brought to zero.

FIG. 18 shows a main routine.

Referring to FIG. 18, first of all, at step 600, the fuel injectionamount Q is calculated based on the output signals from the engine speedsensor 21 and the load sensor 51. Subsequently, it is judged at step 601whether or not the NO_(x) releasing flag has been set. When the NO_(x)releasing flag has not been set, the processing routine goes to step607, at which the duty ratio DUTY is brought to zero, and subsequentlythe processing routine goes to step 605, at which the control of thethrottle valve 53 is carried out. At this time, the duty ratio DUTY iszero, and therefore the throttle valve 53 is retained at the fully openstate. Subsequently, at step 606, the fuel injection processing iscarried out, and the injection amount at this time becomes the injectionamount Q calculated at step 600.

On the other hand, when it is decided at step 601 that the NO_(x)releasing flag has been set, the processing routine goes to step 602, atwhich the injection amount increase value ΔQ is calculated.Subsequently, at step 603, the increase value ΔQ is added to theinjection amount Q, to obtain a new injection amount Q. Subsequently, atstep 604, the duty ratio DUTY is calculated. Subsequently, at step 605,the throttle valve 53 is opened to the opening degree determined by theduty ratio DUTY, and subsequently, at step 606, the fuel is injectedfrom the fuel injector 11 according to the injection amount Q calculatedat step 603.

In the embodiment shown in FIG. 19, a reducing agent supply valve 60 isdisposed in the exhaust pipe 17, which this reducing agent supply valve60 is connected with a reducing agent tank 62 via a supply pump 61. Theoutput port 36 of the electronic control unit 30 is connected to thereducing agent supply valve 60 and the supply pump 61 via the drivingcircuits 63 and 64, respectively. In the reducing agent tank 62, ahydrocarbon such as gasoline, isoctane, hexane, heptane, light oil,kerosine, or the like or a hydrocarbon such as butane, propane, or thelike which can be stored in the state of a liquid is filled.

In this embodiment, usually the air-fuel mixture in the combustionchamber 3 is burned under an excess air state, that is, in a state wherethe average air-fuel ratio is lean. At this time, the NO_(x) dischargedfrom the engine is absorbed into the NO_(x) absorbent 18. When theNO_(x) should be released from the NO_(x) absorbent 18, the supply pump61 is driven and, at the same time, the reducing agent supply valve 60is opened, whereby the hydrocarbon filled in the reducing agent tank 62is supplied from the reducing agent supply valve 60 to the exhaust pipe17 for a predetermined time, for example, about 5 seconds to 20 seconds.The amount of supply of the hydrocarbon at this time is determined sothat the air-fuel ratio of the exhaust gas flowing into the NO_(x)absorbent flowing into the NO_(x) absorbent 18 becomes rich.Accordingly, at this time, the NO_(x) is released from the NO_(x)absorbent 18.

FIG. 20 shows a routine for executing the NO_(x) releasing processing,which routine is executed by interruption at every predetermined timeinterval.

Referring to FIG. 20, first of all, at step 700, a result obtained byadding ΣNE to the present engine speed NE is defined as ΣNE.Accordingly, this ΣNE indicates the cumulative value of the engine speedNE. Subsequently, at step 701, it is judged whether or not thecumulative engine speed ZNE is larger than the predetermined value SNE.This predetermined value SNE indicates a cumulative engine speed fromwhich it is estimated that the NO_(x) in an amount of, for example, 50%of the NO_(x) absorption ability of the NO_(x) absorbent 18 is absorbedtherein. When ΣNE≦SNE, the processing cycle is completed, and whenΣNE>SNE, that is, when it is estimated that the NO_(x) in an amount of50% of the NO_(x) absorption ability of the NO_(x) absorbent 18 isabsorbed therein, the processing routine goes to step 702. At step 702,it is judged whether or not the exhaust temperature T is lower than thepredetermined value T₁, for example, 200° C. When T<T₁, the processingcycle is completed, and when T≧T₁, the processing routine goes to step703, at which the supply pump 61 is driven for a predetermined time, forexample, about 5 seconds to 20 seconds. Subsequently, at step 704, thereducing agent supply valve 60 is opened for a predetermined time, forexample, about 5 seconds to 20 seconds, and subsequently, at step 705,the cumulative engine speed ΣNE is brought to zero.

As mentioned before, when the temperature is lowered, the NO_(x)absorbent 18 becomes not able to absorb the NO_(x). However, in all ofthe embodiments mentioned heretofore, the exhaust gas is always flowsinto the NO_(x) absorbent 18 during the operation of the engine, andtherefore the NO_(x) absorbent 18 is retained at a relatively hightemperature. Accordingly, it becomes possible to cause the NO_(x)generated during the engine operation to be absorbed in the NO_(x)absorbent 18 well.

We claim:
 1. An exhaust purification device of an internal combustion engine including an exhaust passage and means for controlling an air-fuel ratio of exhaust gas from the engine, comprising an NO_(x) absorbent disposed within the exhaust passage, wherein the NO_(x) absorbent absorbs NO_(x) included in the exhaust flowing into the NO_(x) absorbent when the air-fuel ratio control means makes the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent lean and releases an absorbed NO_(x) when an oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent is lowered, and wherein, at all times while the engine is running, the exhaust gas continuously flows into the NO_(x) absorbent so that the NO_(x) absorbed in the NO_(x) absorbent while the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is lean is released from the NO_(x) absorbent when the oxygen concentration of the exhaust gas flowing into the NO_(x) absorbent is lowered.
 2. An exhaust purification device of an internal combustion engine according to claim 1, wherein the air-fuel ratio control means operates to release NO_(x) absorbed in the NO_(x) absorbent by making the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent rich.
 3. An exhaust purification device of an internal combustion engine according to claim 1, wherein the air-fuel ratio control means operates to release NO_(x) absorbed in the NO_(x) absorbent by making the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent substantially the stoichiometric air-fuel ratio.
 4. An exhaust purification device of an internal combustion engine according to claim 1, wherein the air-fuel ratio control means maintains the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent lean for a first predetermined time and decreases the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent for a second predetermined time so as to release the NO_(x) from the NO_(x) absorbent, wherein the first predetermined time is at least 50 times as long as the second predetermined time.
 5. An exhaust purification device of an internal combustion engine according to claim 1, wherein NO_(x) is absorbed into the NO_(x) absorbent when the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is more than 18.0.
 6. An exhaust purification device of an internal combustion engine according to claim 1, wherein the NO_(x) absorbent contains at least one substance selected from alkali metals comprising potassium, sodium, lithium, or cesium, alkali earth metals comprising barium or calcium, rare earth metals comprising lanthanum and yttrium and contains platinum.
 7. An exhaust purification device of an internal combustion engine according to claim 1, wherein the NO_(x) absorbent comprises a composite oxide of barium and copper.
 8. An exhaust purification device of an internal combustion engine according to claim 1, wherein the air-fuel ratio control means controls the air-fuel ratio of an air-fuel mixture formed in an engine combustion chamber so that the absorption of NO_(x) into the NO_(x) absorbent and the release of NO_(x) from the NO_(x) absorbent are controlled by controlling the air-fuel ratio of the air-fuel mixture formed in the engine combustion chamber.
 9. An exhaust purification device of an internal combustion engine according to claim 8, wherein said air-fuel ratio control means makes the air-fuel ratio of the air-fuel mixture formed in the combustion chamber lean when the NO_(x) should be absorbed into the NO_(x) absorbent and makes the air-fuel ratio of the air-fuel mixture formed in the combustion chamber is not lean when the NO_(x) should be released from the NO_(x) absorbent.
 10. An exhaust purification device of an internal combustion engine according to claim 9, wherein the internal combustion engine comprises a gasoline engine and said air-fuel ratio control means controls the absorption of NO_(x) into the NO_(x) absorbent and the releasing of NO_(x) from the NO_(x) absorbent by controlling the fuel amount supplied to the engine.
 11. An exhaust purification device of an internal combustion engine according to claim 10, wherein said air-fuel ratio control means maintains the air-fuel ratio of the air-fuel mixture formed in the combustion chamber at almost a constant lean air-fuel ratio of more than 18.0 when the NO_(x) should be absorbed into the NO_(x) absorbent.
 12. An exhaust purification device of an internal combustion engine according to claim 10, further comprising memory means which stores in advance the amount of fuel determined in accordance with the operation state of the engine, said air-fuel ratio control means determines the amount of fuel supplied to the engine based on the fuel amount stored in said memory means.
 13. An exhaust purification device of an internal combustion engine according to claim 10, further comprising memory means which stores in advance the basic fuel amount determined in accordance with the operation state of the engine and an air-fuel ratio sensor which is provided in the exhaust passage of the engine and detects the air-fuel ratio of the exhaust gas flowing in the exhaust passage, said air-fuel ratio control means corrects the basic fuel amount so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio by a feedback correction coefficient varied in accordance with the output signal of said air-fuel ratio sensor.
 14. An exhaust purification device of an internal combustion engine according to claim 13, wherein said air-fuel ratio control means corrects the basic fuel amount so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio by the feedback correction coefficient when the NO_(x) should be absorbed into the NO_(x) absorbent and, at the same time, corrects said feedback correction coefficient by a learning coefficient so that said feedback correction coefficient fluctuates around a reference value, and said air-fuel ratio control means fixes the feedback correction value to said reference value when the NO_(x) should be released from the NO_(x) absorbent and, at the same time, determines the amount of fuel to be supplied to the engine based on the learning coefficient and the basic fuel amount.
 15. An exhaust purification device of an internal combustion engine according to claim 9, wherein the internal combustion engine comprises a diesel engine equipped with a fuel injector which injects the fuel into the combustion chamber and a throttle valve disposed in the intake passage of the engine; and said air-fuel ratio control means controls the absorption of NO_(x) into the NO_(x) absorbent and the releasing of NO_(x) from the NO_(x) absorbent by controlling the amount of injection from the fuel injector and the opening degree of throttle valve.
 16. An exhaust purification device of an internal combustion engine according to claim 15, wherein said air-fuel ratio control means increases the injection amount and decreases the throttle valve opening degree when the NO_(x) should be released from the NO_(x) absorbent.
 17. An exhaust purification device of an internal combustion engine according to claim 1, further comprising air-fuel ratio control means which controls the air-fuel ratio of the exhaust gas discharged from the engine combustion chamber and flowing into the NO_(x) absorbent in the exhaust passage of the engine, and the absorption of NO_(x) into the NO_(x) absorbent and the releasing of NO_(x) from the NO_(x) absorbent are controlled by controlling the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent by said air-fuel ratio control means.
 18. An exhaust purification device of an internal combustion engine according to claim 17, wherein said air-fuel ratio control means makes the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent lean when the NO_(x) should be absorbed into the NO_(x) absorbent, while makes the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent the stoichiometric air-fuel ratio or rich when the NO_(x) should be released from the NO_(x) absorbent.
 19. An exhaust purification device of an internal combustion engine according to claim 18, wherein said air-fuel ratio control means supplies a reducing agent to the interior of the exhaust passage of the engine when the NO_(x) should be released from the NO_(x) absorbent.
 20. An exhaust purification device of an internal combustion engine according to claim 19, wherein said reducing agent is made of a hydrocarbon.
 21. An exhaust purification device of an internal combustion engine according to claim 20, wherein said hydrocarbon comprises at least one member selected from gasoline, isoctane, hexane, heptane, butane, propane, light oil, and kerosine.
 22. An exhaust purification device of an internal combustion engine according to claim 1, wherein the air-fuel ratio control means lowers the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent during a first predetermined set-up period of time so as to release the NO_(x) from the absorbent when a period of time during which the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent has previously been made lean and, during which NO_(x) has been absorbed into the NO_(x) absorbent, exceeds a predetermined second set-up period of time.
 23. An exhaust purification device of an internal combustion engine according to claim 22, wherein said NO_(x) releasing control means makes the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent the stoichiometric air-fuel ratio or rich when the NO_(x) should be released from the NO_(x) absorbent.
 24. An exhaust purification device of an internal combustion engine according to claim 22, wherein the NO_(x) releasing control means is provided with NO_(x) amount estimation means for estimating the amount of NO_(x) absorbed into the NO_(x) absorbent, and said NO_(x) releasing control means decides that said second set-up period has elapsed when the amount of NO_(x) estimated by the NO_(x) amount estimation means exceeds a preliminarily determined set-up amount.
 25. An exhaust purification device of an internal combustion engine according to claim 24, wherein said NO_(x) amount estimation means decides that the amount of NO_(x) absorbed in the NO_(x) absorbent exceeds said set-up amount when a cumulative value of an engine speed exceeds a preliminarily determined set-up value.
 26. An exhaust purification device of an internal combustion engine according to claim 24, wherein said NO_(x) amount estimation means decides that substantially all of the NO_(x) absorbed in the NO_(x) absorbent has released when the air-fuel ratio of the air-fuel mixture formed in the engine combustion chamber is not lean for a predetermined time or more.
 27. An exhaust purification device of an internal combustion engine according to claim 22, wherein said second set-up period is substantially less than 20 seconds.
 28. An exhaust purification device of an internal combustion engine according to claim 22, wherein said NO_(x) releasing control means is provided with a temperature sensor for detecting a temperature of the exhaust gas flowing into the NO_(x) absorbent, and said NO_(x) releasing control means is provided with prohibition means which prohibits the lowering of the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent even if the period for which the NO_(x) is absorbed into the NO_(x) absorbent exceeds said first set-up period when the temperature of the exhaust gas flowing into the NO_(x) absorbent becomes lower than a limit temperature at which the NO_(x) can be absorbed by the NO_(x) absorbent.
 29. An exhaust purification device of an internal combustion engine according to claim 28, wherein said NO_(x) releasing control means immediately lowers the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent when the temperature of the exhaust gas flowing into the NO_(x) absorbent becomes higher than said limit temperature after the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent is lowered by said prohibition means.
 30. An exhaust purification device of an internal combustion engine according to claim 1, wherein a catalyst which can reduce at least the NO_(x) is disposed in the exhaust passage of the engine downstream of the NO_(x) absorbent.
 31. An exhaust purification device of an internal combustion engine according to claim 30, wherein said catalyst comprises a three-way catalyst.
 32. An exhaust purification device of an internal combustion engine-according to claim 1, wherein a catalyst which can purify the unburnt HC and CO is disposed in the exhaust passage of the engine upstream of the NO_(x) absorbent.
 33. An exhaust purification device of an internal combustion engine according to claim 32, wherein said catalyst comprises a three-way catalyst. 